Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/634

 Crag. Of the first great change produced by the emergence of land to the south previous to the formation of the Red Crag we have scanty evidence. Traces of a Crag of the age probably either of our Coralline Crag or of the Crag noir exists in Touraine; so that until that period there had been communication with southern seas and an interchange of species*. The elevation of the Wealden dome brought to the surface beds of early Crag or of Diestien age, portions of which still exist in our North Downs at an elevation of from 500 to 600 feet† ; and a like elevated tract, capped by beds of the same age, is prolonged into France and Belgium. This formed a barrier separating the southern and northern sea areas, and so isolating the fauna of the Coralline Crag, that, with the lower temperature of the sea resulting from the exclusion of currents from the south and the setting in of others from the north, a great part of that fauna died out. It is a case of extinction by change of conditions, and not by time.

This accounts for the disappearance in the Red Crag, noticed long since by Mr. Searles Wood, of so large a number of the southern genera of shells which characterize the Coralline Crag ; while the descent of the more northern genera continues, with little loss, accompanied by the introduction from time to time of new species of northern forms.

A considerable number of the species which disappeared from our area at the period of the Crag continued to exist further south in the Atlantic and Mediterranean. Other species, under favourable conditions of the low temperature at great depths, survived in the mid- Atlantic, where their existence remained unknown until they were recovered by the deep-sea dredging so successfully carried on of late years. As many as 93 species of the Coralline Crag have been found at greater or less depths in the southern seas ; and of these, 17 met with at depths of from 1000 to 7000 feet had not been before met with living‡. In the same way there are 65 species of the Red and Norwich Crags now found ranging to great depths ; but of these, 39 lived at the time of the Coralline Crag ; and of 4 of these not before known, 2 are Coralline-Crag species.

The presence of northern and arctic species does not, however, necessitate a severe climate ; for cold currents may give a northern facies to the sea-fauna, while the land may retain the mean temperature due to geographical position. Beyond the introduction of more northern forms of shells in the Red and Norwich Crags, there is nothing to indicate a great increase of cold. None of the blocks of the Coralline Crag have been found drifted far from the Sutton islet. The porphyry boulder at the base of the Coralline Crag exceeds in dimensions any other foreign block either in the Red or the Norwich Crag. In the Chillesford Clay itself nothing but large

many of the Bryozoa are common to the Coralline Crag and Douay beds,
 * Only a few shells of the Faluns of Touraine passed into our Crag area ; but

† Quart. Journ. Geol. Soc. vol. xiv. pp. 322 et seqq.

‡ Quart. Journ. Geol. Soc. vol. xxvii. p. liv.